This is the source of the WebVTT specification.
Contributions can be made via the W3C Text Tracks Community Group.
You can file new issues from the specification itself. There are also old bugs reported in W3C BugZilla.
A commit hook ascertains that commits to the master branch automatically updates the spec.
This spec is generated using bikeshed.
To generate a CG draft, run:
$ bikeshed spec
To generate a WD snapshot, run e.g.:
$ ./snapshot.sh WD 2015-10-31